MATLAB Answers

Find returns empty with inconsistent size

4 views (last 30 days)
Rub Ron
Rub Ron on 26 Aug 2021
Commented: Walter Roberson on 26 Aug 2021
I have a variable x, that is a vector with a variable number of elements. When applying the function
find(x>0), for:
find([-1 -1 -1 -1 -1 -1]>0)
ans =
1×0 emptydouble row vector
find([-1 -1]>0)
ans =
1×0 emptydouble row vector
find([-1]>0)
ans =
[]
The problem I have is that when x has only one element, and the size of the output is not consistent. For instance, I would like to get an empty with the same size as "1×0 empty double row vector".
Any suggestions?

Sign in to answer this question.

Accepted Answer

Walter Roberson
Walter Roberson on 26 Aug 2021
Edited:Walter Roberson on 26 Aug 2021
Ran in:
find([-1 -1 -1 -1 -1 -1]>0)
ans = 1×0 emptydoublerow vector
find([-1 -1 -1 -1 -1 -1].'>0)
ans = 0×1 emptydoublecolumn vector
find([-1 -1]>0)
ans = 1×0 emptydoublerow vector
find([-1 -1].'>0)
ans = 0×1 emptydoublecolumn vector
find([-1]>0)
ans = []
size(ans)
ans = 1×2
0 0
So when there is 1 row, the result has 0 columns. When there is 1 column, the result has 0 rows. When there is 1 row and 1 column (scalar), the result has 0 rows and 0 columns.
2 Comments
Show 1 older comment
Walter Roberson
Walter Roberson on 26 Aug 2021
There are no options to find() that can make that behaviour happen. You can, however, do
idx = find([-1]>0);
ifisempty(idx); idx = zeros(1,0);end

Sign in to comment.

More Answers (1)

Awais Saeed
Awais Saeed on 26 Aug 2021
Edited:Awais Saeed on 26 Aug 2021
This command is for vectors and matrices. find(X > 0) returns the indices of the array X where elements are greater than zero. If none is found, it returns an empty matrix. Now using this command on scalers ( [-1] in your case) does not make any sense.
However, if you want 1×0 empty double row vector then you have to check whether X is a scaler or not. If X is a scaler, then do
X = -1:-2% this will be an empty vector and will give you 1×0 empty double row vector
Use
isscalar()
yo check if X is a scaler ot not
3 Comments
Show 2 older comments
Walter Roberson
Walter Roberson on 26 Aug 2021
Ran in:
x = [-1]
x = -1
if(isscalar(x))
result = x : x - 1
else
result = find(x>0)
end
result = 1×0 emptydoublerow vector
x = [+1]
x = 1
if(isscalar(x))
result = x : x - 1
else
result = find(x>0)
end
result = 1×0 emptydoublerow vector
x = [-inf]
x = -Inf
if(isscalar(x))
result = x : x - 1
else
result = find(x>0)
end
result = NaN
In that second case, result should not be empty. In the third case, result should be 1x0 not NaN. Your code x:x-1 is presuming an outcome and is using odd operators to try to enforce it.

Sign in to comment.

Categories

Find more onLogicalinHelp CenterandFile Exchange

Tags

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!